Thermal Nanoquakes: Terahertz Frequency Surface Rayleigh Waves in Diamond Nanocrystals

TL;DR

This paper reveals that thermally induced Rayleigh surface phonons dominate the vibrational dynamics of nanodiamonds, causing surface quakes and explaining the linear scaling of low-energy vibrational states, with implications for quantum sensing and thermal management.

Contribution

It provides the first combined atomistic simulation and experimental analysis of THz surface waves in nanocrystals, elucidating their role in vibrational properties and surface dynamics.

Findings

Thermally induced Rayleigh surface phonons cause surface quakes in nanodiamonds.

Surface displacement ratios exceed those of large earthquakes by 10^5 times.

Rayleigh waves coexist with confined phonons, soft modes, and other surface vibrations.

Abstract

Mechanical THz vibrations in nanocrystals have recently been harnessed for quantum sensing and thermal management. The free boundaries of nanocrystals introduce new surface wave solutions, analogous to the seismic waves on Earth, yet the implications of these surface waves on nanocrystals have remained largely unexplored. Here, we use atomistic molecular dynamics simulations and experimental neutron spectroscopy to elucidate these THz-scale features in nanodiamond. Our key insight is that thermally induced Rayleigh surface phonons, which have a low group velocity and an amplitude that decays exponentially away from the surface, are responsible for the previously observed but unexplained linear scaling of the low-energy vibrational density of states in nanocrystals. Large thermal atomic displacements, relative to the nanoparticle radius, induce perpetual surface quakes, even at ambient conditions. Normalised to the radius, the surface displacement ratio in diamond nanocrystals exceeds that of the largest recorded earthquakes by a factor of $10^{5\pm1}$. We explicate how these dramatic Rayleigh waves coexist with other distinctive features including confined lattice phonons, soft surface modes, the acoustic gap, Love waves, and Lamb modes, thereby offering a complete framework for the vibrational dynamics of nanocrystals.